For rubber products such as pneumatic tires and industrial belts, in order to reduce the weights of the products and to improve the durability of the products, a high tensile strength and an excellent fatigue resistance is required for a steel cord used as a reinforcement. These days, in order to achieve the same tire strength as the existing conditions while reducing the amount of steel cords used, it is required that the tensile strength of each steel filament of the steel cord as the reinforcement be increased.
In order to meet such demands, many researches and reports from a variety of viewpoints have been made, and it is known to be important that the ductility of a steel wire be increased to attempt to increase the tensile strength. In order to achieve an increase in the tensile strength, an evaluation of properties such as the ductility of a steel wire is therefore performed. For example, when properties such as the ductility of a carbon steel wire are evaluated, conventionally, a technique by which an evaluation is performed by using a cross sectional hardness distribution has been employed.
For example, Patent Document 1 discloses a high strength steel wire which can achieve a high strength by allowing the hardness distribution in a high carbon steel wire to satisfy the condition:0.960≦HV≦1.030at R=0, R=0.8, R=0.95(when the radius of the steel wire is r0 and the distance between any point on the steel wire and the center of the steel wire is r, R=r/r0, and when the hardness at the point where R=0.5 is HV0.5 and the hardness at the point R is HVR, HV=HVR/HV0.5). The Patent Document 2 reports that an ultrahigh strength and a high tenacity can be obtained by making a Vickers hardness distribution on the cross section of a wire of a high carbon steel wire substantially flat from the surface to inside except for the center portion having a fourth of the diameter of the wire.
A variety of production processes are proposed for realizing a high ductility and a high fatigue resistance in a final wet wire drawing process. For example, the Patent Document 3 reports that each reduction of area in the final wire drawing process is adjusted in a predetermined range by a processing strain applied to a material wire of steel cords, for the purpose of obtaining a high quality steel wire also by a general purpose steel cord. The Patent Document 4 reports that a wire drawing process is performed in the final wire drawing process, with each die having a constant reduction of area of about 15% to about 18%, for the purpose of obtaining a high tensile strength steel wire having a high torsional ductility.